The inventive concepts described herein relate to a semiconductor memory device, and more particularly, relate to a nonvolatile memory device and an operating method thereof.
A semiconductor memory device is fabricated using semiconductor materials, such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP), and so on. Semiconductor memory devices are classified as volatile memory devices or nonvolatile memory devices.
Generally, volatile memory devices lose stored contents at power-off. Examples of volatile memory devices include random access memory (RAM), static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), and the like. Nonvolatile memory devices retain stored contents even at power-off. Examples of nonvolatile memory devices include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable and programmable ROM (EEPROM), flash memory, phase-change RAM (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM), ferroelectric RAM (FRAM), and the like.
As a nonvolatile memory device, program and erase operations of a flash memory device, for example, may be performed to vary a threshold voltage of a memory cell. As electrons are moved by a strong electric field applied to a thin oxide film, the threshold voltage of the memory cell may be varied. As the number of program and erase operations of the flash memory device increases, electrons may be trapped by the thin oxide film. This may cause an increase in the threshold voltage of the memory cell. Thus, the number of fail bits indicating that data is not erased in an erase operation may increase, deteriorating reliability.